Tolerance reflects the distance that two points can be apart and still be considered the same (for example, to accommodate rounding errors). The tolerance value must be a positive number greater than zero.
[OSUGR] . Tolerances are used in some geometric test, mostly for equality between coordinates. Such tests occur frequently in spatial queries with conditions such as "on boundary" or "just touching". A tolerance can also be used when formulating step distances for parametric curve interpolation.